The present invention is directed to automatic circuit testing and in particular to capacitive tests for open circuits.
One of the initial steps in testing electronic circuits that comprise a number of integrated circuits mounted on a circuit board is to insure that proper interconnections have been made among the integrated circuits. This includes testing for short circuits between paths on the printed-circuit board and for open circuits between integrated-circuit connection pins and the printed-circuit-board conductive paths to which they are supposed to be soldered or otherwise conductively connected. Open circuits are a particular problem when some of the integrated circuits are surface-mount-type devices, whose connection pins tend to be small and closely spaced. Accordingly, considerable work has been directed to developing tests that detect and identify open circuits.
Certain of the tests employ the digital driver/sensors provided in large numbers on many of the more-elaborate automatic test-equipment systems. Other tests, which can be used not only on elaborate systems but also on less-expensive systems, which do not have as many driver/sensors and sophisticated "back driving" capabilities, are of the capacitive type. In tests of this type, an inexpensively generated analog signal is applied to a circuit path, and a probe having a conductive plate is placed in close proximity to the package of an integrated circuit whose pin should be electrically connected to that printed-circuit path. The proximity of the conductive probe to the package results in capacitance between the probe and certain conductors in the package to which the pin is connected. If the integrated circuit pin is properly connected to the circuit path, the signal applied to the path will appear, by capacitive coupling, on the probe. A judgment as to whether the desired connection exists can therefore be made in accordance with the magnitude of the probe signal that results from the capacitive coupling. (This method can also be used reciprocally, i.e., by using capacitive coupling to apply the signal to the integrated circuit and using conventional conductive probing to sense the resultant signal on the circuit path.)
In the past, this method has presented some probing difficulty, but the type of probe described in the co-pending U.S. patent application of Paul R. Freve for a Probe for Capacitive Open-Circuit Tests, filed on even date herewith and assigned to the assignee hereof, significantly ameliorates the probing problem. However, this leaves another problem, which is the time-consuming nature of the open-circuit testing: before in-circuit or functional tests of the electronic circuit can begin, all or nearly all device pins must be tested for possible open circuits, and the number of such pins on a given board can be quite large.